The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal flexible connecting members and nerves of the spine. The spinal column includes a series of vertebrae stacked one on top of the other, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces experienced by the spinal column. A vertebral canal containing the spinal flexible connecting members and nerves is located behind the vertebral bodies.
The more than twenty vertebrae are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae and connect to the lower spine, which comprises the five lumbar vertebrae. The base of the spine is the sacral bone (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. In spite of these complexities, the spine is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. A motion segment is defined between each adjacent vertebra that may exhibit extension, flexion and twisting.
There are many types of spinal column disorders including scoliosis (abnormal lateral curvature of the spine), kyphosis (abnormal forward curvature of the spine, usually in the thoracic spine), excess lordosis (abnormal backward curvature of the spine, usually in the lumbar spine), spondylolisthesis (forward displacement of one vertebra over another, usually in a lumbar or cervical spine) and other disorders caused by abnormalities, disease, or trauma, such as ruptured or slipped discs, degenerative disc disease, fractured vertebra, and the like. Patients that suffer from such conditions usually experience extreme and debilitating pain as well as diminished range of motion and nerve function. These spinal pathologies may threaten the critical elements of the nervous system housed within the spinal column.
A variety of systems and surgical procedures have been disclosed in the art to alleviate the symptoms of these and other spinal pathologies. One of the most common surgical procedures today is arthrodesis, or spine fusion, of one or more spine segments. Spine fusion is used to treat many spinal disorders, including kyphosis, spondylolisthesis, and lordosis. Clinical success varies considerably, depending upon technique and indications, and consideration must be given to the concomitant risks and complications. For example, it has been shown that spine fusion decreases function by limiting the range of motion for patients in flexion, extension, rotation, and lateral bending. Furthermore, it has been shown that spine fusion creates increased stresses and, therefore, accelerated degeneration of adjacent non-fused motion segments. Also, the fusion device, whether artificial or biological, may migrate out of the fusion site.
Instead of fusing sections of the spine, various devices have been implanted into the spine to stabilize the spine without completely restricting movement. These flexible spinal stabilization methods may not result in complete spinal fusion. Some systems include implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine that is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods that are aligned along the axis of the bones and which are attached to the spinal column by hooks coupled to the lamina or to the transverse processes. Implants may also be attached by screws or attachment members inserted through the pedicles.
One posterior stabilization method includes spinal immobilization utilizing pedicle screws and rods. Other spinal systems may include a variety of other procedures and apparatuses for attending spinal problems and pain. However, there is a continual need for alternative systems and devices for stabilization of the spine.